In magnetic resonance imaging there is a general tendency to obtain acceptable images within shorter periods of time. For this reason the sensitivity encoding method called “SENSE” has recently been developed by the Institute of Biomedical Engineering and Medical Informatics, University and ETH Zürich, Switzerland. The SENSE method is based on an algorithm which acts directly on the image as detected by the coils of the magnetic resonance apparatus and in which subsequent encoding steps can be skipped and hence an acceleration of the signal acquisition for imaging by a factor of from two to three can be obtained. Crucial for the SENSE method is the knowledge of the sensitivity of the coils arranged in so called sensitivity maps. In order to accelerate this method there are proposals to use raw sensitivity maps which can be obtained through division by either the “sum-of-squares” of the single coil references or by an optional body coil reference (see e.g. K. Pruessmann et. al. in Proc. ISMRM, 1998, abstracts pp. 579, 799, 803 and 2087). In fact the SENSE method allows for a decrease in scan time by deliberately undersampling k-space, i.e. deliberately selecting a Field-of-View (FOV) that is smaller than the object to be acquired. From this undersampling fold-over artefacts are obtained which can be resolved or unfolded by the use of the knowledge of a set of distinct coils having different coil sensitivity patterns. The undersampling can be in either one of both phase-encoding directions.
It is commonly known that the main magnet of the Philips NT with the Intera 1.5 T magnet (Philips Medical Systems, Best, Netherlands) has a quit sharp distinction between the inner region in which the main magnetic field is homogeneous and the outer region in which the main magnetic field is completely inhomogeneous. For the Intera 1.5 T magnet the inner region of high homogeneity is larger than the usual extent of the human body in the left-to-right direction which is orthogonal to the elongate direction of the magnet. The polynomial expansion of the main magnetic field is zero up to a high-order, which means that there is a quick transition between the inner region or volume of high homogeneity and the outer region of high inhomogeneity.
Other known MR systems have another design of their main magnets, which have a more gradual transition between the high homogeneity region and the region in which the main magnetic field is completely inhomogeneous, i.e. where fields are not sufficient homogeneous for any RF refocusing whatsoever. Such magnets are being mentioned here as of “lower order”.
For the set-up of SENSE it is required to provide a sensitivity map of the coils which is done by means of a large voxel gradient echo imaging or fast field echo scan (FFE), which is also known as coarse calibration scan (COCA). This scan is sensitive to signal loss due to intra-voxel dephasing in case of moderate magnetic field inhomogeneities. For this reason up to now the SENSE method is practically only feasible with the Philips MR system because of the high homogeneity of the main magnetic field.
In U.S. Pat. No. 5,910,728 a magnetic resonance imaging apparatus and technique exploits spatial information inherent in a surface coil array to increase MR image acquisition speed, resolution and/or field of view. The MR signal from a combination of coils having an aggregate sinusoidal and cosinusoidal spatial sensitivity profile have an information content somewhat different from that of the usual coil signal. By separating out one or more collected signals corresponding to pure spatial harmonics, these may be used to fill a larger portion of the data space than is done conventionally. Partial signals are thus acquired simultaneously in the component coils of the array and formed into two or more signals corresponding to orthogonal spatial representations. In a Fourier embodiment, lines of the k-space matrix required for image production are formed using a set of separate linear combinations of the component coil signals to substitute for spatial modulations normally produced by phase encoding gradients. The combined MR signal from the inhomogeneous coils is thus shifted in k-space by a predetermined amount dependent from the spatial frequency of the inhomogeneous coil sensitivity. This k-space shift has precisely the same form as the phase-encoding shift produced by evolution in a y gradient. This method is specifically designed for SMASH. However, it does not give any further indication to solve the problem of using SENSE with designs of main magnet of lower order as discussed above.
It is an object of the present invention to allow the application of SENSE with a main magnet with a more gradual transition between the region with a homogeneous magnetic field and the region with a inhomogeneous magnetic field, i.e. with magnets of lower order.
This object of the invention are achieved by a method as defined in claim 1. The invention is further related to an apparatus as defined in claim 6 and to a computer program product as defined in claim 7.
The present invention has the main advantage that main magnets with a less sharp distinction between the inner region of high homogeneity and the outer region of complete inhomogeneity, i.e. magnets of lower order, now can be used also for the above mentioned SENSE method.
These and other advantages of the invention are disclosed in the dependent claims and in the following description in which an exemplified embodiment of the invention is described with respect to the accompanying drawings. Therein shows: